Help - I need an education on head tube angles please
 

Hi all,

I have been looking at some vintage "sport" touring bikes lately and have noticed something as I have poured over the fame geometry project. Note: I am asking this question specific to smaller frames 21" or 53cm. I make that point because in larger frames this change did not happen.

In the 70's it seems that head tube angles were 71 degrees for bikes in this size range. Somewhere in the early 80s the switch to 73 degree angles occurred. Trek seemed to do this pretty consistently. I should also point out that it seems the seat tube angles stayed the same at 73 no matter the size of the bike.

Can anyone explain why? What are the real world ride difference?

Head Tube angle is one aspect of steering geometry that, when combined with fork "rake" contribute to a bike's handling characteristics.
In general, the steeper the head angle (say 73.5-75) the more responsive the steering will tend to be (at the expense of some stability).
The slacker the angle, the more stable the bike will tend to be (but less responsive).

The combination of rake and head angle are key contributors to "trail", which is a more useful stat than either of the two taken on their own.

This link may help: http://www.phred.org/~josh/bike/trail.html

Thanks - I will take a look at that article - many of the bikes I am looking at have a rake of 5.5 CM with a head tube angle of 71.5. I guess my real question is how the handling would compare to a more current bike with a rake of 5.5 and a head tube angle of 73. The latter seems to be much more the "standard".

73 is still a pretty modest angle, but is steep enough that it will feel more responsive (for a given rake) than the 71.

E_merlin, Another contributer to a bicycle's stability is the wheelbase. The shorter wheelbase became, the slacker the HT angle became (to increase trail) to prevent a nervous riding bicycle.

Another reason for the slacker headtube angle on smaller frames is to simply fit a commonly used size wheel/tire that doesn't interfere with the downtube or the rider's toes. Sometimes the smallest of frames were accomodated with a smaller front wheel, which would cause another trail calculation.

Brad

Trail is measured by length (imperial or metric) and rake is in degrees. If the head tube angle is different, but the trail is the same it is because of the fork's offset and both bikes would be equally stable.

Brad

That was very helpful.

Thanks to all - this is staring to make more sense especially for toe overlap. So with a 44.5 cm rear on one of these bikes you really have a cruiser. Getting close to touring dimensions it seems.

The wheel base on one I am considering is like 102.7.

Here's a handy calculator to figure trail for various head tube angles, fork offsets, and wheel sizes:

http://yojimg.net/bike/web_tools/trailcalc.php

Neal

I'm guessing that, when the head tube angles went from 71 degrees to 73 degrees, the top tube lengths increased. As Bradtx noted, the slack head tube angles on smaller frames was so that they could use 700c (or 27") wheels in the front. But to increase the head tube angle and still use 700c wheels, you have to increase the length of the top tube. The real world effect of this is (1) needing to use shorter stems or (2) havong a more stretched out ridng position. It also makes it harder for women to use the frame, as the average woman has a proportionally shorter upper body than does the average man.

All of this reflects a general trend over the past 40 years. It's easiest to see in racing bikes. If you look at photos of pro racers in the 1950s, their riding positions are much more upright than they are today. Charly Gaul and Francisco Bahamontes were extreme examples of this - both were pure climbers and each won the Tour de France once. But as you move forward in time, positions got lower and more stretched out, and the top tubes got longer to accommodate these positions. Since the trend has increasingly been to make bikes sold to the public look like what the pros ride, the stuff we buy followed the trend, smaller frames included.

There has been some change lately as we Baby Boomers age and don't have the flexibility to bend over so much. Hence somewhat more upright "sports touring" designs, which look an awful lot like older racing geometry frames.

Hope this helps.

Thanks, Neal. That site is now bookmarked.

Brad

Thanks very much -that makes perfect sense.

Great discussion - I wonder how many of the old frame builders used tables v/s eye balls and trail set ups to set their head set angles - I am always surprised at how comfortable some of the older bike frames ride - I suspect that with newer materials some of the old rules change - And who can afford experimentation... Good thread...

This article was interesting was well...

http://www.calfeedesign.com/tech-pap...bike-handling/

Here is also a good thread here on the subject with many good links

http://www.bikeforums.net/showthread...ht=ideal+trail